The present invention relates to compact or miniprinters, and more particularly to printer assemblies to be incorporated in apparatus such as electronic calculators and the like.
In conventional compact printers or miniprinters, such as that disclosed in the specification of Japanese Patent Laid Open Number 56-63471, or U.S. Pat. No. 4,458,589, a drum is driven by a stepper motor through a gear train. One or more types are selectively struck from inside the type drum to perform printing. The mechanism carries out printing by selectively suspending movement of the types.
A difficulty associated with this conventional design is that even if the stepper motor is stopped, the type drum may still rotate or vibrate because of backlash in the gear train. Thus, the type drum may not come to a standstill at a given position or at the desired time. If typing is performed when such an unstable condition exists, the characters printed in each column may not be uniform and the linearity or alignment of the characters may be poor.
If printing is not executed until the transient motions subside, the printing cycle becomes very long and printing speed is low. In addition, when the vibrations are very large, there is a danger that the proper type will not engage with the hammer, so that mistyping and misplacement of characters may result.
In these conventional arrangements, the motor gear and motor have a shaft separate and apart from that of the type drum. As a result, the printer requires a relatively wide space. The motor gear, the motor and the type drum all require respective shafts and bearings. Thus, many different components are required including separate screws for attaching the motor, and the cost is increasd. These conventional printers require excess energy to drive both the gears and the type drum and therefore a great deal of motor energy is consumed and much noise is generated by the gears.
In addition, in these conventional printers, it is necessary to provide a signal which indicate that the type drum is in a standard or "home" position in order to properly synchronize the printing with the position of the drum. If this signal is not maintained in its proper "phase" with respect to rotation of the drum due to backlash or wear, then mistyping will result. Further, discrepancies in timing between paper feed and motor rotation or play in a power transmission section of a paper feed mechanism in a conventional printer can cause the extremely serious difficulty of paper feed during typing.
Accordingly, it is desirable to provide a compact or miniprinter which can be incorporated into a portable electronic device which overcomes these shortcomings of the prior art devices.